The invention deals with a method of controlling a hydrostatic/mechanical power split transmission, which comprises an adjustable hydrostat unit, a multiple shaft summing planetary gearbox for combining the mechanical path and hydrostatic path, and a step gearbox connected downstream in order to form gear change ranges which adjoin one another at synchronization points and which are passed through by means of adjusting the hydrostat unit, it being the case that, at each change from an old range to a next range, firstly, when the synchronization speed is reached, the command to engage the clutch for the next range is given, then the transfer of the load from the clutch for the old range to the clutch for the next range is performed by adjusting the hydrostat unit, and only then is the command to disengage the clutch for the old range given.
In such transmissions, the clutches are generally switched hydraulically. Changes of range are performed in several steps, several clutches and the hydrostat being actuated in a specific sequence and depending on state signals or completion messages. The control needs a feedback signal for the actual position of specific clutches. The actuation signal is not sufficient, since there could certainly also be a defective clutch or a pressure line could be leaky, so that the clutch is not engaged, in spite of the "completion message". In the case of a claw coupling the two halves of the coupling could also be exactly claw to claw, or a selector lever could have become stuck.
EP 280 757-A1 discloses, in an embodiment of such a transmission, the practice of connecting the pressure signal line to the clutches to a pressure sensor or pressure switch via branch lines. In the case of this known transmission, in the course of a range change following the command to engage the clutch assigned to the next range and, once more following the command to disengage the clutch assigned to the preceding range, the feedback signal from the respective pressure sensor or pressure switch is awaited before the next step of the range change is executed.
On the one hand, this double wait prolongs the duration of a range change and, during sharp acceleration, leads to a brief drop in the acceleration, particularly if the pressure switches are connected to their clutch via long lines. On the other hand, the failure risk of pressure switches of this type, which occur in large numbers for this purpose (in the case of a transmission with four ranges, six clutches are generally needed), is considerable. Although pressure sensors are more secure, truly reliable sensors, or ones which can be checked in operation, are expensive. The consequence of a wrong state signal is at least that range changes are no longer possible.
These problems arise irrespective of the type of clutches used (these may be claw couplings, tooth couplings, synchronization clutches or multiplate clutches) and irrespective of the type of step gearbox connected downstream of the summing planetary gearbox. It therefore arises even in the case of a transmission according to U.S. Pat. No. 5,207,736 where, instead of the pressure of the clutch actuation, the position of the selector cuffs can be interrogated by means of sensors.
It is thus the object of the present invention to specify a control method which accelerates the sequence of range changing, while completely maintaining the operational reliability, and manages without pressure switches or pressure sensors and is thus more cost-effective.